Gernot Buth

Direct Catalytic Asymmetric Three-Component Kabachnik–Fields Reaction†

As mimics of a-amino acids, a-amino phosphonates have great promise as antibacterial and anti-HIV agents as well as protease inhibitors. Consequently, their enantioselective synthesis, in particular by catalytic enantioselective hydrophosphonylation of preformed imines, has attracted considerable interest. Both chiral metal complexes as well as organic catalysts such as Jacobsen4s chiral thiourea derivatives, chiral phosphoric acid derivatives, and quinine have been used with success. Despite these achievements however, a direct catalytic asymmetric Kabachnik– Fields reaction, has to our knowledge not been described before. Here we show that racemic a-branched aldehydes, in the presence a new chiral phosphoric acid catalyst, directly react with p-anisidine and a phosphite to furnish bbranched a-amino phosphonates highly diastereoselectively and enantioselectively by a dynamic kinetic resolution. Akiyama et al. recently developed an enantioselective hydrophosphonylation of preformed aromatic and unsaturated imines that is catalyzed by chiral binol-derived phosphoric acids pioneered in their laboratories as well as by Terada et al. Our group reported a highly enantioselective reductive amination of a-branched aldehydes by dynamic kinetic resolution (DKR) that is catalyzed by TRIP, a chiral phosphoric acid we have developed previously. We envisioned that an extension of these strategies to the Kabachnik–Fields reaction may be possible, leading directly to b-branched a-amino phosphonates 4 [Eq. (2)]. The analogous b-branched a-amino carboxylic acids have attracted attention in peptidomimetic chemistry as they uniquely restrict the conformational flexibility within a peptide. Our design, however, seemed to be particularly challenging as it combines a dynamic kinetic resolution with the parallel creation of an additional stereogenic center.

A Family of 3d-4f Octa-Nuclear [MnIII4LnIII4] Wheels (Ln = Sm, Gd, Tb, Dy, Ho, Er, and Y): Synthesis, Structure, and Magnetism

We present the syntheses, crystal structures, and magnetochemical characterizations for a family of isostructural [Mn(4)Ln(4)] compounds (Ln = Sm, Gd, Tb, Dy, Ho, Er, and Y). They were prepared from the reactions of formic acid, propionic acid, N-n-butyl-diethanolamine, manganese perchlorate, and lanthanide nitrates under the addition of triethylamine in MeOH. The compounds possess an intriguing hetero-octanuclear wheel structure with four Mn(III) and four Ln(III) ions alternatively arranged in a saddle-like ring, where formate ions act as key carboxylate bridges. In the lattice, the molecules stack into columns in a quasi-hexagonal arrangement. Direct current (dc) magnetic susceptibility measurements indicated the depopulation of the Stark components at low temperature and/or very weak antiferromagnetic interactions between magnetic centers. The zero-field alternating current (ac) susceptibility studies revealed that the compounds containing Sm, Tb, and Dy showed frequency-dependent out-of-phase signals, indicating they are single-molecule magnets (SMMs). Magnetization versus applied dc field sweeps on a single crystal of the Dy compound down to 40 mK exhibited hysteresis depending on temperatures and field sweeping rates, further confirming that the Dy compound is a SMM. The magnetization dynamics of the Sm and Y compounds investigated under dc fields revealed that the relaxation of the Sm compound is considered to be dominated by the two-phonon (Orbach) process while the Y compound displays a multiple relaxation process.

A series of new structural models for the OEC in photosystem II

A new series of MMn(II-III)(4) clusters (M = Na, Ca) has been structurally characterised and their relevance to understanding the oxygen evolving centre of photosystem II is discussed.

Tridecanuclear [MnIII5LnIII8] Complexes Derived from N-tButyl-diethanolamine: Synthesis, Structures, and Magnetic Properties

The synthesis, structures and magnetic properties of a family of heterometallic [Mn(III)(5)Ln(III)(8)(mu(3)-OH)(12)(L(2))(4)(piv)(12)(NO(3))(4)(OAc)(4)](-) (Ln = Pr, 2; Nd, 3; Sm, 4; Gd, 5; Tb, 6) aggregates are reported. The complexes were obtained from the direct reaction of N-(t)butyldiethanolamine (H(2)L(2)) with Mn(OAc)(2) x 4 H(2)O and Ln(NO(3))(3) x 6 H(2)O in the presence of pivalic acid (pivH) in MeCN under ambient conditions. Compounds 2-6 are isomorphous and crystallize in the monoclinic space group P2(1)/n with four molecules in the unit cell. The complexes have a centrosymmetric tridecanuclear anionic core consisting of two distorted inner heterometallic [Mn(III)Ln(III)(3)(mu(3)-OH)(4)](8+) cubane subunits sharing a common Mn vertex flanked by four edge-sharing heterometallic [Mn(III)Ln(III)(2)(mu(3)-OH)(4)](5+) defect cubane units. Complexes 2-6 are the first high-nuclearity 3d-4f aggregates reported to date using (t)Bu-deaH(2) as ligand. These compounds show no evidence of slow relaxation behavior above 1.8 K, which appears to be the consequence of the very weak or non-existent magnetic interactions between the Mn(III) and Ln(III) ions resulting from the particular angles at the bridging oxygens.

Family of MnIII2Ln2(μ4-O) Compounds: Syntheses, Structures, and Magnetic Properties

An isostructural family of tetranuclear aggregates [Mn(III)(2)Ln(2)(O)(Piv)(2)(hep)(4)(NO(3))(4)].MeCN (where Ln = Y(III) (1), Pr(III) (2), Nd(III) (3), Gd(III) (4), Tb(III) (5), Dy(III) (6), Ho(III) (7), and Yb(III) (8)) is reported. They were obtained from the reactions of 2-(2-hydroxyethyl)pyridine (hepH) with a preformed hexanuclear manganese complex, [Mn(6)], and the respective lanthanide salt. The complexes are isomorphous and represent a new heterometallic 3d-4f complex type for this class of ligand. The structural core of 1-8 consists of a distorted Mn(2)Ln(2) tetrahedron with the four metal centers linking through a mu(4)-O(2-) bridging atom. The magnetic properties of all complexes were investigated by variable temperature magnetic susceptibility and magnetization measurements. The magnetic data of all compounds suggest that antiferromagnetic interactions are present between adjacent paramagnetic centers. Complexes 5-7 containing highly anisotropic lanthanide ions (Tb, Dy, and Ho) show slow relaxation of their magnetization.

The supramolecular organization of self-assembling chlorosomal bacteriochlorophyll c, d, or e mimics

Bacteriochlorophylls (BChls) c, d, and e are the main light-harvesting pigments of green photosynthetic bacteria that self-assemble into nanostructures within the chlorosomes forming the most efficient antennas of photosynthetic organisms. All previous models of the chlorosomal antennae, which are quite controversially discussed because no single crystals could be grown so far from these organelles, involve a strong hydrogen-bonding interaction between the 31 hydroxyl group and the 131 carbonyl group. We have synthesized different self-assemblies of BChl c mimics having the same functional groups as the natural counterparts, that is, a hydroxyethyl substituent, a carbonyl group and a divalent metal atom ligated by a tetrapyrrole. These artificial BChl mimics have been shown by single crystal x-ray diffraction to form extended stacks that are packed by hydrophobic interactions and in the absence of hydrogen bonding. Time-resolved photoluminescence proves the ordered nature of the self-assembled stacks. FT-IR spectra show that on self-assembly the carbonyl frequency is shifted by ≈30 cm−1 to lower wavenumbers. From the FT-IR data we can infer the proximal interactions between the BChls in the chlorosomes consistent with a single crystal x-ray structure that shows a weak electrostatic interaction between carbonyl groups and the central zinc atom.

New heterometallic [Mn(III)(4)Ln(III)(4)] wheels incorporating formate ligands.

Two heteroctanuclear wheel complexes with an eight-membered saddle-like ring of [Mn(III)(4)Ln(III)(4)] (Ln = Dy, Gd), prepared by employing formates as key carboxylate bridges, exhibit antiferromagnetic interactions between Mn(III) and Ln(III) centers, and the wheel containing anisotropic Dy(III) ions shows SMM behaviour.

The X-ray-fluorescence facility at ANKA, Karlsruhe: Minimum detection limits and micro probe capabilities

Abstract At the recently constructed synchrotron radiation facility ANKA at Karlsruhe, Germany, a beamline partially dedicated to X-ray fluorescence has been constructed where TXRF and micro probe SRXRF experiments can be performed. The beam line with its retractable double multilayer monochromator is described. First measurements on reference samples allow a preliminary characterization of beam line parameters like exploitable energy range and the energy resolution as well as the sensitivity of the set-up in means of minimum detection limits.

Programmed Metalloporphyrins for Self-Assembly within Light-Harvesting Stacks: (5,15-Dicyano-10,20-bis(3,5-di- tert -butylphenyl)porphyrinato)zinc(II) and Its Push−Pull 15- N , N -Dialkylamino-5-cyano Congeners Obtained by a Facile Direct Amination

The title dicyano compound was synthesized via cyanation and it self-assembles in nonpolar solvents giving red-shifted and broad absorption maxima just as the bacteriochlorophylls which are encountered in the light-harvesting organelles of early photosynthetic bacteria. In the crystal, stacks are formed through a hierarchic combination of pi-stacking and a CN-Zn electrostatic interaction. Push-pull 15-N,N-dialkylamino-5-cyano congeners could be obtained in high yields using a solvent- and catalyst-free direct amination of meso-bromoporphyrins. Importantly, the fluorescence of the self-assembled species due to the very orderly manner in which the chromophores are arranged is not entirely quenched and has a surprisingly long lifetime of over 1 ns. This lends hope of using the trapped energy in biomimetic hybrid solar cells.

Searching for novel crystal forms by in situ high-pressure crystallisation: the example of gabapentin heptahydrate

High-pressure crystallisation of an aqueous solution of the GABA analogue gabapentin at 0.8 GPa resulted in the formation of a previously unknown heptahydrate form, whose structure has been determined by in situ single-crystal X-ray diffraction. The structure and water framework of this unusually highly hydrated small molecule are described in detail.